Assessing the Suitability of Thermally Sprayed Fly ASH Coatings For Marine Structures

Abstract

The current study assesses the suitability of fly ash in both powder and coating forms to cater to marine applications. Due to its rich mineralogy, fly ash (FA), an industrial waste, has been used to combat erosive, corrosive environments. Powder flowability dictates coating properties. In this investigation, firstly, raw FA powder was obtained from a thermal power plant and sieved in various sizes to assess their flowability. Powder’s physical characteristics, such as specific surface area, Blaine’s fineness number, and bulk density, were determined, and their influence on powder flowability was analysed. Of these properties, bulk density affects more. Rietveld refinement was performed on the powder to quantify the phases. The powders had 45.08 ± 11.38% amorphous and 11.00 ± 2.76% of mullite phases. Later, alumina was added between 10 and 50 wt% to FA, and samples were subjected to in situ high-temperature X-ray diffraction at 1150 ℃. A ⁓32.27% rise in Mullite content was observed for 50 wt% alumina, with a ⁓119% decrease in the amorphous phase. Finally, one set of FA without additives coating was plasma sprayed onto a marine-grade steel substrate. The coating showed ⁓17.31 ± 0.6% of mullite and ⁓69.43 ± 0.6% of the amorphous phase, with decent Mechanical properties. Therefore, 50 wt% alumina in FA powder has improved the mullite phase, bulk density (43%), and flowability by decreasing the amorphous phase content. Secondly, fly ash powders were doped with carbon nanotube (CNT) (1 or 2 wt%) powder feedstock produced using four routes: sonication, ball milling without crushing media and ball milling in the presence of ethanol (wet mixing), and spray drying. From the X-ray dot map, wet mixing and spray drying showed a homogenous dispersion of CNT in the FA powder. The flowability of all these powders was quantified with the help of a standard powder feeder. The wet-mixed and spray- dried powders exhibited better flowability. Finally, as-received FA from the power plant was deposited on a Marine grade steel substrate using the plasma-spraying technique to assess the performance of such coatings for erosion and corrosion properties. The coating has exhibited more than 100 % improvement in microhardness. The erosion resistance was improved by ∼11% compared to that of the uncoated sample, which is attributed to the hardness to elastic modulus ratio (H/E) and its unique mineralogy. The minor improvement in erosion resistance was attributed to the icoating’s poor fracture toughness. The erosion study shows that slurry concentration and rotational speeds were the most influential parameters. The scar depth was significantly shallower for FA-coated samples. The corrosion resistance has improved by ∼13.49%, owing to the porous nature of the coating. Therefore, such coatings with appropriate improvements in their properties are expected to alleviate both environmental and industrial challenges.